LM5021: LM5021-2 shutdown using SS pin

Hello Jacob,

Thank you for your interest in the LM5021 flyback controller.

I think you should be able to shut down the LM5021 by grounding SS, but the datasheet is rather vague on this issue. As I see it, pulling SS down will pull COMP low as well, with a 0.55V offset.  When SS is below 0.3V, COMP will be < 0.85V.  Since COMP has an internal 1.25V level-shift, that will ensure that the PWM (-) input voltage = 0V which is below the skip-cycle threshold of 125mV. 

Essentially, pulling SS low will inhibit all PWM output pulses. With SS < 0.3V, the internal states will be set up to begin a soft-start when SS is released.

However, SS low will not completely turn off the controller, it only inhibits PWM, so the IC bias current will continue to drain the VCC cap. Without PWM, there is no AUX power to hold up VIN/VCC, so it will drop to the UVLO threshold and begin a restart cycle to bring VIN back up to 20V, then enable the VCC regulator and attempt to run.  But with SS tied to GND, no PWM can take place so VCC will bleed back down to UVLO again and the cycle repeats until SS is released. Note: restart will be delayed if SS is released before VIN has reached the 20V level.

If this UVLO-cycling (with simple VIN circuit, datasheet page 17) is acceptable, then you’re all set.

If not acceptable, then the alternative is to add a series switch to gate VIN from the AUX winding and open it when necessary. Switching will not stop immediately, but after VCC falls to the UVLO level.  The application circuit in the datasheet (page 19) has a switched VIN (to save bias power) which may be modified to include external on/off control.

Regards,

Ulrich

Thank you, that is very helpful.